The present invention relates to a semiconductor laser diode, and more particularly to the oscillation frequency stabilization of the semiconductor laser diode.
In accordance with recent remarkable improvements in semiconductor laser diode and optical fiber technologies, studies have been conducted on an analog transmission system with direct intensity modulation of the semiconductor laser output. However, due to the coherency of laser output which is intrinsic in the laser output itself and the dispersion of modes in optical fibers, speckle patterns are produced in optical fibers. As such speckle patterns are caused to fluctuate in optical fibers by the chirping of oscillation frequencies at current modulation, "modal noise" is often produced and presents a problem in fiber optic analog transmission systems. In order to achieve a high quality fiber optic analog transmission, stabilization of laser diode oscillation frequency is keenly desired.
Conventionally semiconductor laser diodes fabricated by forming a Fabry-Perot resonator with two cleavage planes, have been used. Electric current is pumped into an active layer sandwiched between a p-type semiconductor layer and an n-type semiconductor layer from two electrodes of a p-type and an n-type, and amplifying light is emitted in the active layer to obtain laser output. However, these semiconductor lasers are defective in that when the concentration of free carriers which are injected into the active layer is changed in order to intensity-modulate the laser output, the optical length of the resonator changes. This is due to the changes in the active layer refractive index which is dependent on the carrier density through free carrier plasma effect and band-to-band optical transitions. In addition, variation of the injected current produces active layer temperature changes, which also results in the refractive index variation. These refractive index variation cause oscillation frequency fluctuation.